1. Field of the Invention
The present invention relates to a laser diode (LD) module in which an LD and an optical fiber are mounted to align the optical axes thereof.
2. Description of the Related Art
FIG. 2 is a cross-sectional view of a conventional LD module. In FIG. 2, the direction of the optical axis is represented by the Z direction, and the direction orthogonal to the Z direction is represented by the XY direction. An LD 1 is supported by an LD holder 2 at welds 11a and 11b. A lens 3 is supported by a lens barrel 4 through press fitting or the like, and the lens barrel 4 and the LD 1 are supported by a lens holder 5 at welds 12a and 12b. One leading end of an optical fiber 6 is supported by a ferrule 7 through press fitting or the like, and the ferrule 7 is supported by a ferrule holder 8 at welds 13a and 13b. The components on the side of the lens holder 5 and the components on the side of the ferrule holder 8 are thereby combined into one unit.
After the optical axes of the lens 3 (and the LD 1) and the optical fiber 6 are aligned, the lens holder 5 and the ferrule holder 8 are connected at welds 14a and 14b, and then, the ferrule holder 8 and a protective casing 9 are connected at welds 15a and 15b. Adjustment of the optical axes is conducted by connecting a photoreceptor to the other end of the optical fiber 6 and finding the position on the XY plane where the light receiving level of the photoreceptor is highest while the LD 1 is in operation. The protective casing 9 is shaped like a cylinder to cover the above-mentioned components 1 to 8, and is provided with a flange portion 9a for attachment. The ferrule holder 8 is provided with a rubber hood 10 for protecting the optical fiber 6.
In the conventional LD module mentioned above, since the protective casing 9 and the ferrule holder 8 forming an outer housing of the LD module are connected at the welds 15a and 15b, the lens holder 5 is required to be moved in the XY direction relative to the ferrule holder 8 on the side of the protective casing 9 in order to align the optical axes of the lens holder 5 and the ferrule holder 8. Therefore, in consideration of the clearance for the lens holder 5 to be moved in the XY direction and positioned inside the protective casing 9, the inner diameter of the protective casing 9 is required to be at least larger than the sum of the diameter of the lens holder 5 and the above-mentioned positioning clearance. Since the LD 1 has the largest diameter of all the components contained in the protective casing 9, the outer diameters of the lens holder 5 for supporting the LD 1 and the protective casing 9 for covering the lens holder 5 are inevitably large, which results into an increased diameter of the LD module. Particularly, this problem of upsizing the LD module is noticeable in the above-mentioned conventional LD module in which the end surface of the optical fiber 6 is cut at a bevel to prevent light emitted from the LD 1 and reflected by the end surface from returning to the LD 1 because a wide adjustment range (clearance) in the XY direction is needed to make such adjustment that the LD light obliquely entering the end surface of the optical fiber 6 is refracted and advanced in the direction of the optical axis of the optical fiber 6.
In such an LD module, the LD 1 and the optical fiber 6 are required to not be displaced due to outside vibrations during operation. For example, the LD module is subjected to a vibration test based on well-known Bellcore (Bell Communications Research) Standards, in which the LD module is swept in the XY and Z directions at 20 G and 2 kHz to 20 kHz. In the above conventional LD module, since vibrations applied to the flange portion 9a of the protective casing 9 are transmitted in order to the protective casing 9, the welds 15a and 15b, the ferrule holder 8, the welds 14a and 14b, and the lens holder 5, the welds 14a and 14b between the ferrule holder 8 and the lens holder 5 are greatly stressed. Therefore, more welds are needed to maintain enough strength to prevent the ferrule holder 8 and the lens holder 5 from displacing with respect to each other after welding, which makes a welding operation troublesome.